1. Field of the Invention
The present invention relates to a liquid crystal display panel.
2. Description of the Related Art
A liquid crystal display panel includes a liquid crystal inserted between the first substrate and the second substrate. Transparent electrodes are formed on the first and second substrates, so that a voltage can be applied to the liquid crystal. In a twisted nematic liquid crystal display panel, alignment films are formed on the first and second substrates, and rubbing is effected to the alignment films in mutually perpendicular directions. The liquid crystal molecules adjacent to the first and second substrates are oriented along the rubbing direction. Between the first and second substrates, the liquid crystal is twisted by 90 degrees from the side of the one substrate toward the side of the other substrate.
In recent years, an active matrix drive has been employed to improve the resolution. The active matrix drive comprises a plurality of small picture electrodes, orthogonal bus lines (gate bus lines and drain bus lines) and active elements such as TFTs provided on one of the substrates. The electrode on the other substrate is a common electrode. By controlling the TFTs, an electric field is formed between a particular picture electrode and the common electrode to produce a display. In the active matrix-driven liquid crystal display device, the region of the picture electrode serves as a pixel (when there is a black matrix, an opening of the black matrix serves as a pixel).
Japanese Unexamined Patent Publication (Kokai) No. 5-5886 discloses an STN liquid crystal panel in which row electrodes are formed on one substrate and column electrodes are formed on the other substrate, and a region where the row electrode overlaps the column electrode serves as a pixel. However, the STN liquid crystal display panel does not employ the active matrix drive. In order to decrease the leakage of light without using black matrix according to this publication, after the alignment film is rubbed, only the portions outside the pixels are irradiated with ultraviolet rays, so that a pretilt angle of the liquid crystal increases in the portions outside the pixels. The invention of the present application utilizes the irradiation of ultraviolet rays, but the present invention is directed to solve a problem that stems from the employment of the active matrix drive, and the object of the present invention is different from the object of irradiation of ultraviolet rays of the above publication.
As liquid crystal display panels employing the active matrix drive made larger, there arises a problem concerning uniformity in the display of the liquid crystal display panels. When a voltage is continuously applied to the whole surface of, for example, a 21-inch liquid crystal display panel, for extended periods of time, a white uneven pattern appears in the display near the corners of the display screen.
The object of the present invention is to provide a liquid crystal display panel in which an uneven pattern does not appear in the display even when the panel is fabricated in a larger size.
According to the present invention, the liquid crystal display panel comprises first and second substrates opposed to each other; a liquid crystal inserted between the first and second substrates; the first substrate having bus lines, active elements, picture electrodes and a first alignment film; the second substrate having a common electrode and a second alignment film; and wherein the first and second alignment films are treated such that the liquid crystal molecules adjacent to each alignment film are oriented in a predetermined direction, and at least one of the first and second alignment films is at least partly treated such that its surface condition is changed to restrain the movement of the liquid crystal.
In analyzing an uneven pattern that appears in the display when a voltage is applied, it has been found that the thickness of the cell gap of a portion of the liquid crystal display device where the uneven pattern appears in the display is larger than the thickness of the cell gap of other portions. When no voltage is applied, an uneven pattern does not appear and there is no change in the thickness of the cell gap. From this fact, it is thought that the uneven pattern is caused by the fact that liquid crystal moves while the voltage is being applied. According to the present invention, therefore, treatment is carried out to change the surface condition of the alignment film in order to restrain the movement of the liquid crystal. As a result, it is possible to eliminate an uneven pattern in the display.
Preferably, the treatment for changing the surface condition of the alignment film is effected by irradiation of ultraviolet rays. In this case, it is desired that the irradiation of ultraviolet rays is effected maintaining energy of not smaller than 2500 mJ. Or, the treatment for changing the surface condition of the alignment film is effected by changing the material of the alignment film.
Preferably, the orientation treatment is effected by rubbing. In this case, the orientation treatment is effected after the treatment for changing the surface condition of the alignment film.
Preferably, the treatment for changing the surface condition of the at least one alignment film is effected for both the first and second alignment films. In this case, the treatment for changing the surface condition of the at least one alignment film is effected for part of the first alignment film and for part of the second alignment film. Treated portions of the first alignment film can be brought into alignment with nontreated portions of the second alignment film, and nontreated portions of the first alignment film can be brought into alignment with treated portions of the second alignment film.
Or, the treatment for changing the surface condition of the at least one alignment film is effected for one of the first and second alignment films. In this case, the treatment for changing the surface condition of the at least one alignment film is effected for the whole surface or for only part of the surface of either one of the first alignment film or the second alignment film.
Preferably, the treatment for changing the surface condition of the alignment film is effected for at least either one of the first alignment film or the second alignment film in a striped fashion traversing a plurality of pixel regions.
Preferably, the treatment for changing the surface state of the alignment film is effected for the regions covering the bus lines, and the pixel regions are not subjected to the treatment for changing the surface state of the alignment film.
Preferably, a spacer is inserted between the first substrate and the second substrate.